There are a couple ways one can build a koi pond using the koi tiles found on this site. Straight edged ponds are one option, another more abstract pond option is also available.
To build a pond with straight egdes, such as a square or rectangular pond, first place the koi tiles (or the bamboo fountain) where you'd like the koi to appear in your pond. I also like to add a border of flowers or plants for a more formal look (see the picture to the right).
Then once you have your border and koi tiles where you like them, fill in the blank space with the plain blue water tile. Voila! A lovely koi pond. Keep in mind, you needn't restrict yourself to a square or a rectangle. Try making other straight-edged shapes, or maybe build your pond around other objects or even around the whole house. (Don't forget to leave a path for the Sims to get out though!)
To go for a more abstract shape pond requires that you use the water_tool cheat (pre Hot Date), or the Pond Tool in build mode with Hot Date or later.
using the water_tool cheat (pre Hot Date)
Place your koi tiles, fountain, or plants (or any other object for that matter) where you want them, as in the picture to the left. Then use the water_tool cheat to fill in the water. To use the cheat type Ctrl+Shift+C (or Command+Shift+C for Macintosh) to bring up the cheat window. Type in water_tool and hit Enter/Return. This activates the water_tool cheat. Now just click where you want to put the water. It is important when using the cheat to place the objects first, and then fill in the water.
using the Pond Tool (Hot Date or later)
Use the Pond Tool in Build Mode to fill in the water where you want your pond. Then place your koi tiles or fountain where you want them, as in the picture to the left. It is important to put the water down first with this tool and then place the objects on the water.
Voila! A lovely koi pond. Again, don't forget to leave places for your Sims to walk.